Frontline Plus is on sale at Drugstore.com for 18%-27% off [drugstore.com] (depending on dosage)

In addition, if you search Bing.com for “Frontline Plus” Drugstore.com pops up in the sponsored results (right hand column) with 20% Cash Back.

Free Shipping on purchases over $49, so any 6-pack should qualify. You may also choose to mix-and-match 3 Packs if you have multiple pets, to crack the free shipping threshold.

Directions
1. Log in or sign up for Bing cashback [bing.com]
2. Search Bing for “Frontline Plus”
3. Click on 20% CB sponsored result (should bring up the 20% Bing screen while transferring you to Drugstore.com)
4. Search Drugstore.com for “Frontline”
5. Pick your Frontline.
6. Check out.